Solid-state image sensor and method of manufacturing the same

ABSTRACT

A method of manufacturing a solid-state image sensor, includes forming a first isolation region of a first conductivity type in a semiconductor layer having first and second surfaces, the forming the first isolation region including first implantation for implanting ions into the semiconductor layer through the first surface, forming charge accumulation regions of a second conductivity type in the semiconductor layer, performing first annealing, forming an interconnection on a side of the first surface of the semiconductor layer after the first annealing, and forming a second isolation region of the first conductivity type in the semiconductor layer, the forming the second isolation region including second implantation for implanting ions into the semiconductor layer through the second surface. The first and second isolation regions are arranged between the adjacent charge accumulation regions.

This application is a continuation of U.S. patent application Ser. No.15/606,233, filed May 26, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/939,215, filed Nov. 12, 2015 and issuing as U.S.Pat. No. 9,704,905, which is a divisional of U.S. patent applicationSer. No. 14/099,449, filed Dec. 6, 2013 and issuing as U.S. Pat. No.9,231,019.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a solid-state image sensor and a methodof manufacturing the same.

Description of the Related Art

Japanese Patent Laid-Open No. 2009-111118 relates to a solid-state imagesensor, and describes a manufacturing method of reliably isolatingsignal charges between photoelectric conversion regions. Themanufacturing method forms a first pixel isolation region by doping animpurity into a semiconductor substrate, forms a first epitaxial growthlayer on the surface of the semiconductor substrate, and forms a secondpixel isolation region to extend through the first epitaxial growthlayer and contact the first pixel isolation region.

Japanese Patent Laid-Open No. 2006-93587 relates to a solid-state imagesensor, and describes a method of manufacturing the solid-state imagesensor, which prevents color mixing due to an increase in number ofpixels per unit and suppresses generation of a dark current in anaccumulation layer. The manufacturing method forms a photodiode in ann-type semiconductor layer arranged on an n-type semiconductor via asilicon oxide film, forms a p-type pixel isolation region to surroundthe photodiode, and forms a front-surface side p⁺ accumulation layer.After that, the manufacturing method forms a back-surface side p⁺accumulation layer by implanting ions into the n-type semiconductorlayer from its back surface side.

If an isolation region for isolating charge accumulation regions formedon a semiconductor layer from each other is formed by only ionimplantation through one (to be referred to as an ion implantationsurface hereinafter) of the two surfaces of the semiconductor layer, thewidth of the isolation region can be wider away from the ionimplantation surface. This is because high implantation energy isrequired to implant ions into a region away from the ion implantationsurface (that is, a deep region), thereby widening a region into whichions are implanted. This phenomenon in which the width of the isolationregion becomes wider away from the ion implantation surface prevents thedensity of charge accumulation regions or pixels from increasing.

SUMMARY OF THE INVENTION

The present invention provides a manufacturing method advantageous inincreasing the density of charge accumulation regions or pixels, and asolid-state image sensor having a structure advantageous inmanufacturing by the manufacturing method.

One of aspects of the present invention provides a method ofmanufacturing a solid-state image sensor, comprising: forming a firstisolation region of a first conductivity type in a semiconductor layerhaving a first surface and a second surface, the forming the firstisolation region including first implantation for implanting ions intothe semiconductor layer through the first surface; forming a pluralityof charge accumulation regions of a second conductivity type differentfrom the first conductivity type in the semiconductor layer; performingfirst annealing after the first implantation; forming an interconnectionon a side of the first surface of the semiconductor layer after thefirst annealing; and forming a second isolation region of the firstconductivity type in the semiconductor layer, the forming the secondisolation region including second implantation for implanting ions intothe semiconductor layer through the second surface, the secondimplantation being performed after the forming the interconnection,wherein the first isolation region and the second isolation region arearranged between two adjacent charge accumulation regions of theplurality of charge accumulation regions.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing the arrangement of asolid-state image sensor according to the first or second embodiment;

FIG. 2 is a plan view schematically showing the arrangement of thesolid-state image sensor according to the first or second embodiment;

FIGS. 3A and 3B are views for explaining a method of manufacturing thesolid-state image sensor according to the first embodiment;

FIGS. 4A and 4B are views for explaining the method of manufacturing thesolid-state image sensor according to the first embodiment;

FIGS. 5A and 5B are views for explaining the method of manufacturing thesolid-state image sensor according to the first embodiment;

FIGS. 6A and 6B are views for explaining the method of manufacturing thesolid-state image sensor according to the first embodiment;

FIGS. 7A and 7B are views for explaining a method of manufacturing thesolid-state image sensor according to the second embodiment;

FIGS. 8A and 8B are views for explaining the method of manufacturing thesolid-state image sensor according to the second embodiment;

FIG. 9 is a view for explaining the method of manufacturing thesolid-state image sensor according to the second embodiment;

FIG. 10 is a view for explaining a solid-state image sensor and a methodof manufacturing the solid-state image sensor according to the thirdembodiment;

FIG. 11 is a view for explaining a solid-state image sensor and a methodof manufacturing the solid-state image sensor according to the fourthembodiment; and

FIG. 12 is a view for explaining a solid-state image sensor and a methodof manufacturing the solid-state image sensor according to the fifthembodiment.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings.

FIG. 1 is a sectional view schematically showing the arrangement of asolid-state image sensor 100 according to the first embodiment of thepresent invention. FIG. 2 is a plan view schematically showing thearrangement of the solid-state image sensor 100 according to the firstembodiment of the present invention. FIG. 1 is a sectional view takenalong a line X-X′ in FIG. 2.

The solid-state image sensor 100 includes a semiconductor layer 101having a first surface F1 and a second surface F2, a plurality of chargeaccumulation regions 103 arranged in the semiconductor layer 101, andisolation regions 120 and 130 arranged in the semiconductor layer 101.The isolation regions 120 and 130 are arranged in the semiconductorlayer 101 to isolate the plurality of charge accumulation regions 103from each other. The isolation regions 120 and 130 are impuritysemiconductor regions formed by ion implantation, and form potentialbarriers, respectively.

The solid-state image sensor 100 includes a plurality of microlenses171. Note that the plurality of microlenses 171 are arrayed so that onemicrolens 171 is assigned to each charge accumulation region pairconsisting of two charge accumulation regions 103. The solid-state imagesensor 100 is configured to be able to individually read out signalscorresponding to charges accumulated in each of the two chargeaccumulation regions 103 of the charge accumulation region pair. Thisarrangement can be used for focus detection by a phase-differencedetection method. Furthermore, the solid-state image sensor 100 can beconfigured to be able to individually read out a signal corresponding tothe sum of the charges accumulated in each of the two chargeaccumulation regions 103 of the charge accumulation region pair. Thesignal corresponding to the sum of the charges accumulated in each ofthe two charge accumulation regions 103 of the charge accumulationregion pair corresponds to the signal of one pixel.

The isolation region 120 is an inter-pair isolation region which isarranged between the charge accumulation region pair and another chargeaccumulation region pair and forms a potential barrier. The isolationregion 130 is an intra-pair isolation region which is arranged betweenthe two charge accumulation regions 103 of the charge accumulationregion pair and forms a potential barrier. The potential barrier formedby the intra-pair isolation region 130 is smaller than that formed bythe inter-pair isolation region 120. This arrangement allows chargesoverflowing from one of the two charge accumulation regions 103 of onecharge accumulation region pair to migrate to the other chargeaccumulation region, and is advantageous in preventing the migration ofcharges from the charge accumulation region pair to another chargeaccumulation region pair. This contributes to widening the dynamic rangeand reducing color mixing.

As a method of making the potential barrier formed by the intra-pairisolation region 130 smaller than that formed by the inter-pairisolation region 120, for example, there are the following first tothird methods. Two or more of the first to third methods may be used incombination.

In the first method, the intra-pair isolation region 130 is formed by animpurity semiconductor region with a first number of steps, and theinter-pair isolation region 120 is formed by an impurity semiconductorregion with a second number of steps, the first number being smallerthan the second number.

In the second method, the impurity concentration of the intra-pairisolation region 130 is made lower than that of the inter-pair isolationregion 120.

In the third method, the width of the intra-pair isolation region 130 ina direction along the first surface F1 is made smaller than that of theinter-pair isolation region 120 in the direction along the first surfaceF1.

The inter-pair isolation region 120 can include a first isolation region121 and a second isolation region 122. The first isolation region 121can be formed by implanting ions into the semiconductor layer 101through the first surface F1. The second isolation region 122 can beformed by implanting ions into the semiconductor layer 101 through thesecond surface F2.

The intra-pair isolation region 130 can include a first isolation region131 and a second isolation region 132. The first isolation region 131can be formed by implanting ions into the semiconductor layer 101through the first surface F1. The second isolation region 132 can beformed by implanting ions into the semiconductor layer 101 through thesecond surface F2. For example, the first isolation region 131 and thesecond isolation region 132 do not contact each other. The intra-pairisolation region 130 may be formed by an impurity semiconductor regionwith the first number of steps smaller than the second number of steps,thereby preventing the first isolation region 131 and the secondisolation region 132 from contacting each other.

The solid-state image sensor 100 can include a front surface pinninglayer 105 arranged between the first surface F1 and the chargeaccumulation regions 103. The solid-state image sensor 100 can alsoinclude a back surface pinning layer 107 arranged to be adjacent to thesecond surface F2. Note that the isolation regions 120 and 130, thefront surface pinning layer 105, and the back surface pinning layer 107are formed by impurity semiconductor regions of a first conductivitytype. The semiconductor layer 101 and the charge accumulation regions103 can be formed by impurity semiconductor regions of a secondconductivity type different from the first conductivity type. If thefirst conductivity type is p-type, the second conductivity type isn-type, and vice versa.

The solid-state image sensor 100 can include floating diffusions 106 inthe semiconductor layer 101. The floating diffusions 106 can be formedby impurity semiconductor regions of the second conductivity type. Atransfer gate 141 can transfer charges accumulated in the chargeaccumulation region 103 to the floating diffusion 106 through a channelformed in the semiconductor layer 101. In addition, the solid-stateimage sensor 100 can include reset transistors for respectivelyresetting the potentials of the floating diffusions 106, andamplification transistors for respectively outputting signalscorresponding to the charges transferred to the floating diffusions 106to a vertical signal line.

The solid-state image sensor 100 can include a multiplayer wiringstructure 140 on the side of the first surface F1. The multiplayerwiring structure 140 can include gate electrodes such as the transfergates 141, wiring patterns 143, an insulating film 145, contact plugs(not shown), and via plugs (not shown). Moreover, the solid-state imagesensor 100 can include an antireflection film 161, light-shielding films163, an insulating film 165, and a color filter layer 167 on the side ofthe second surface F2. The solid-state image sensor in which themultiplayer wiring structure 140 is arranged on one side (the side ofthe first surface) of the semiconductor layer 101 and the microlenses171 is arranged on the other side (the side of the second surface) ofthe semiconductor layer 101 can be called a back-side illuminationsolid-state image sensor. However, the present invention is not limitedto this.

The solid-state image sensor 100 can include a support substrate 151 onthe side of the multiplayer wiring structure 140. The support substrate151 supports the multiplayer wiring structure 140, the semiconductorlayer 101, and the like.

A method of manufacturing the solid-state image sensor 100 according tothe first embodiment will be described below with reference to FIGS. 3A,3B, 4A, 4B, 5A, 5B, 6A, 6B, and 1. In a process shown in FIG. 3A, asemiconductor substrate 101′ such as a silicon substrate is prepared,element isolations such as a well and STI (Shallow Trench Isolation) areformed in the semiconductor substrate 101′, and first isolation regions121 and 131 are also formed. The semiconductor substrate 101′ willbecome a semiconductor layer 101 later. The first isolation regions 121and 131 can be formed by performing an ion implantation process for thesemiconductor substrate 101′ through its first surface F1 at least once(typically, a plurality of times). The first isolation regions 121 and131 can be formed by impurity regions of the first conductivity type, asdescribed above. If the first conductivity type is p-type, for example,the first isolation regions 121 and 131 can be formed by implantingboron into the semiconductor substrate 101′ at 1.5 MeV, 1 MeV, 600 keV,300 keV, 100 keV, and 50 keV. In some implementations, ions can beimplanted into only the inter-pair isolation region 120 of theinter-pair isolation region 120 and the intra-pair isolation region 130in some of a plurality of ion implantation processes.

In a process shown in FIG. 3B, charge accumulation regions 103, a frontsurface pinning layer 105, floating diffusions 106, gate electrodes suchas transfer gates 141, and a diffusion region of transistors are formedin the semiconductor substrate 101′. Note that the gate electrodes suchas the transfer gates 141 are formed on the first surface F1 via a gateinsulating film. As described above, the charge accumulation regions 103and floating diffusions 106 are formed by impurity regions of the secondconductivity type and the front surface pinning layer 105 is formed byan impurity region of the first conductivity type. If the firstconductivity type is p-type, for example, the front surface pinninglayer 105 can be formed by implanting boron into the semiconductorsubstrate 101′ at 10 keV.

After performing the ion implantation processes for forming the impuritysemiconductor regions such as the first isolation regions 121 and 131, afirst annealing process for recovering crystal defects due to the ionimplantation can be performed. The first annealing process can beperformed by, for example, an FA (Furnace Annealing) method using anelectric furnace or an RTP (Rapid Thermal Annealing) method.

In a process shown in FIG. 4A which is performed after the firstannealing process, a multiplayer wiring structure 140 is formed on thefirst surface F1 of the semiconductor substrate 101′.

A process shown in FIG. 4B is an optional process in which a supportsubstrate 151 is bonded to the multiplayer wiring structure 140. On thesurface of the multiplayer wiring structure 140, a planarized insulatingfilm is typically exposed. The support substrate 151 is a substrate suchas a silicon substrate or glass substrate, and typically has aplanarized surface. The support substrate 151 can be bonded to themultiplayer wiring structure 140 in, for example, a vacuum or inert gasatmosphere. It is possible to increase the bonding strength byirradiating the surface of the multiplayer wiring structure 140 and thesurface of the support substrate 151 with a plasma before the bondingprocess. Alternatively, before the bonding process, the surface of themultiplayer wiring structure 140 and the surface of the supportsubstrate 151 may be activated by a chemical treatment.

Although the surface of the support substrate 151 is directly bonded tothe surface of the multiplayer wiring structure 140 in the above bondingprocess, an adhesive material may be used for bonding. Benzocyclobutene,for example, can be used as the adhesive material, and allows bonding atabout 250° C.

In a process shown in FIG. 5A, the semiconductor substrate 101′ isthinned by processing the side of a second surface F2′ of thesemiconductor substrate 101′, thereby forming a semiconductor layer 101having a processed second surface F2. The semiconductor substrate 101′can be thinned by, for example, grinding, polishing, CMP (ChemicalMechanical Polishing), or etching. If the semiconductor layer 101 is asilicon layer, it preferably has a thickness of, for example, 2 to 10μm. In this case, the semiconductor layer 101 absorbs 80% or more oflight of a wavelength range of 400 to 700 nm which includes thewavelength range of visible light and wavelengths in its neighborhood.

In a process shown in FIG. 5B, second isolation regions 122 and 132 areformed. The process shown in FIG. 5B is performed after the processshown in FIG. 4A. The second isolation regions 122 and 132 can be formedby performing an ion implantation process for the semiconductor layer101 through its second surface F2 at least once (typically, a pluralityof times). As described above, the second isolation regions 122 and 132can be formed by impurity regions of the first conductivity type. If thefirst conductivity type is p-type, for example, the second isolationregions 122 and 132 can be formed by implanting boron into thesemiconductor layer 101 at 600 keV, 300 keV, 100 keV, and 50 keV. Thiscan form an inter-pair isolation region 120 including the firstisolation region 121 and the second isolation region 122, and anintra-pair isolation region 130 including the first isolation region 131and the second isolation region 132. In some implementations, ions canbe implanted into only the inter-pair isolation region 120 of theinter-pair isolation region 120 and the intra-pair isolation region 130in some of a plurality of ion implantation processes.

The number of times of ion implantation for forming the second isolationregion 122 is preferably smaller than that for forming the firstisolation region 121. Alternatively, the dimension of the secondisolation region 122 in the depth direction is preferably smaller thanthat of the first isolation region 121 in the depth direction.Furthermore, the number of times of ion implantation for forming thesecond isolation region 132 is preferably smaller than that for formingthe first isolation region 131. Or, the dimension of the secondisolation region 132 in the depth direction is preferably smaller thanthat of the first isolation region 131 in the depth direction. This isbecause heat is preferably, selectively applied near the second surfaceF2 in a second annealing process for recovering crystal defects due toion implantation for forming the second isolation regions 122 and 132and a back surface pinning layer 107 (to be described below).

In a process shown in FIG. 6A, the back surface pinning layer 107 isformed by implanting ions near the second surface F2. If the firstconductivity type is p-type, for example, the back surface pinning layer107 can be formed by implanting boron into the semiconductor layer 101through the second surface F2 at 10 keV. After that, the secondannealing process for recovering crystal defects due to the ionimplantation for forming the second isolation regions 122 and 132 andthe back surface pinning layer 107 is performed. In the second annealingprocess, a method different from that used in the first annealingprocess can be used. At this time, since the multiplayer wiringstructure 140 has already been formed on the side of the first surfaceF1, heat is selectively applied near the second surface F2 in the secondannealing process so as to prevent the temperature of the wiringpatterns 143 from reaching the melting point.

The second annealing process can be performed by, for example, a methodof irradiating the second surface F2 with light. More specifically, thesecond annealing process can be performed by, for example, a laserannealing method or flash lamp annealing method. If the laser annealingmethod is applied, the second surface F2 is irradiated with a laser beamusing a 308-nm (XeCl) excimer laser for about 100 nsec.

The second annealing process may be performed after forming anantireflection film 161 and before forming light-shielding films 163 (tobe described later). In this case, ion implantation for forming the backsurface pinning layer 107 may be performed after forming theantireflection film 161 (and before performing the second annealingprocess). If ion implantation for forming the back surface pinning layer107 is performed after forming the antireflection film 161, theantireflection film 161 can function as a buffer layer for preventingchanneling in ion implantation.

In a process shown in FIG. 6B, the antireflection film 161 is formed onthe second surface F2 of the semiconductor layer 101. The antireflectionfilm 161 can be formed by, for example, a silicon oxide film and asilicon nitride film. The antireflection film 161 can have, for example,a stacked structure including a silicon oxide film having a thickness of5 nm and a silicon nitride film having a thickness of 50 nm or a stackedstructure including a silicon oxide film having a thickness of 5 nm, asilicon nitride film having a thickness of 50 nm, and a silicon oxidefilm having a thickness of 50 nm. Note that the antireflection film 161is not limited to them, and can adopt any structure having anantireflection function.

In the process shown in FIG. 6B, the light-shielding films 163 areformed on the antireflection film 161. The light-shielding films 163 canbe made of, for example, aluminum or tungsten. Note that thelight-shielding films 163 are optional components.

A description will be provided with reference to FIG. 1. In a processshown in FIG. 1, an insulating film (planarized film) 165 is formed onthe light-shielding films 163 and the antireflection film 161, a colorfilter layer 167 is formed on the insulating film 165, and thenmicrolenses 171 are formed on the color filter layer 167.

A manufacturing method according to the second embodiment of the presentinvention will be described below with reference to FIGS. 7A, 7B, 8A,8B, and 9. A method of obtaining a thinned semiconductor layer 101 inthe second embodiment is different from that in the first embodiment.Details not mentioned in the second embodiment conform to the firstembodiment unless a mismatch occurs.

In a process shown in FIG. 7A, an SOI (Silicon On Insulator) substrateis prepared. The SOI substrate includes a buried insulating layer 201 ona handle substrate 203, and includes a semiconductor layer 101 on theburied insulating layer 201. In the process shown in FIG. 7A, a backsurface pinning layer 107 is formed in the semiconductor layer 101 byimplanting ions into the semiconductor layer 101 through its firstsurface F1. For example, the back surface pinning layer 107 can beformed to contact the buried insulating layer 201. Note that whenmanufacturing an SOI substrate, a back surface pinning layer 107 may beformed in a semiconductor layer 101. Furthermore, in the process shownin FIG. 7A, element isolations such as a well and STI (Shallow TrenchIsolation) are formed in the semiconductor layer 101, and firstisolation regions 121 and 131 are formed. After performing an ionimplantation process for forming impurity semiconductor regions such asthe first isolation regions 121 and 131, a first annealing process forrecovering crystal defects due to the ion implantation can be performed.The first annealing process can be performed by, for example, an FA(Furnace Annealing) method using an electric furnace or an RTP (RapidThermal Annealing) method.

In a process shown in FIG. 7B, charge accumulation regions 103, a frontsurface pinning layer 105, floating diffusions 106, gate electrodes suchas transfer gates 141, a diffusion region of transistors, and the likeare formed in the semiconductor layer 101.

In a process shown in FIG. 8A, a multiplayer wiring structure 140 isformed on the first surface F1 of the semiconductor layer 101. Theprocess shown in FIG. 8A is performed after the first annealing process.A process shown in FIG. 8B is an optional process in which a supportsubstrate 151 is bonded to the multiplayer wiring structure 140.

In a process shown in FIG. 9, the handle substrate 203 and the buriedinsulating layer 201 are removed (that is, the SOI substrate is thinnedto leave the semiconductor layer 101). This process corresponds to thethinning process shown in FIG. 5A in the first embodiment. The handlesubstrate 203 and the buried insulating layer 201 can be removed by, forexample, etching the handle substrate 203 using the buried insulatinglayer 201 as an etching stop layer, and then etching the buriedinsulating layer 201. Note that if the buried insulating layer 201 has astructure usable as an antireflection film 161, a process of removingthe buried insulating layer 201 and a process of forming anantireflection film 161 can be omitted.

The subsequent processes are the same as those shown in FIGS. 5B, 6A,6B, and 1 in the first embodiment.

The third embodiment of the present invention will be described belowwith reference to FIG. 10. Details not mentioned in the third embodimentconform to the first or second embodiment unless a mismatch occurs. Inthe first and second embodiments, one microlens 171 is assigned to eachcharge accumulation region pair including two charge accumulationregions 103. In the third embodiment, one microlens 171 is assigned toeach charge accumulation region 103. An isolation region 120 is arrangedbetween charge accumulation regions 103, thereby forming a potentialbarrier. The isolation region 120 can include a first isolation region121 and a second isolation region 122. The first isolation region 121can be formed by implanting ions into a semiconductor layer 101 throughits first surface F1. The second isolation region 122 can be formed byimplanting ions into the semiconductor layer 101 through its secondsurface F2.

The number of times of ion implantation for forming the second isolationregion 122 is preferably smaller than that for forming the firstisolation region 121. Alternatively, the dimension of the secondisolation region 122 in the depth direction is preferably smaller thanthat of the first isolation region 121 in the depth direction.

A solid-state image sensor and a method of manufacturing the solid-stateimage sensor according to the third embodiment are the same as those inthe first embodiment except that the isolation regions 120 are arrangedinstead of the isolation regions 130 in the first embodiment and onemicrolens 171 is assigned to each charge accumulation region 103.

The fourth embodiment of the present invention will be described belowwith reference to FIG. 11. Details not mentioned in the fourthembodiment conform to the first or second embodiment unless a mismatchoccurs.

In the fourth embodiment, a plurality of microlenses 171 are arrayed sothat one microlens 171 is assigned to each charge accumulation regionpair including two charge accumulation regions 103. In the fourthembodiment, a first isolation region 120 includes a first isolationregion 121 and a second isolation region 122 which are interconnected bya connection surface IF. The first isolation region 121 is arrangedbetween a first surface F1 and the connection surface IF to contact theconnection surface IF. The second isolation region 122 is arrangedbetween a second surface F2 and the connection surface IF to contact theconnection surface IF. The width of the first isolation region 121 islarger on the side of the connection surface IF than on the side of thefirst surface F1 and/or the width of the second isolation region 122 islarger on the side of the connection surface IF than on the side of thesecond surface F2. This can reduce the possibility that the firstisolation region 121 and the second isolation region 122 are separatedfrom each other due to an alignment error in a lithography process.

The above-described structure of the first isolation region 121 can berealized by performing ion implantation into the semiconductor layer 101through the first surface F1 a plurality of times. That is, high ionimplantation energy is required to implant ions into a position awayfrom the first surface F1 (a deep position), thereby widening, in thelateral direction (a direction parallel to the first surface F1), aregion into which ions are implanted. For example, ion implantation canbe performed a plurality of times at different energy levels using maskshaving the same opening. Similarly, the above-described structure of thesecond isolation region 122 can be realized by performing ionimplantation into the semiconductor layer 101 through the second surfaceF2 a plurality of times.

FIG. 12 illustrates the fifth embodiment of the present invention. Thefifth embodiment is the same as the fourth embodiment except thatisolation regions 120 are arranged instead of the isolation regions 130in the fourth embodiment, and one microlens 171 is assigned to eachcharge accumulation region 103.

As an application of the slid-state image sensor according to each ofthe above embodiments, a camera incorporating the solid-state imagesensor will be exemplified. The camera conceptually includes not only adevice whose principal purpose is photographing but also a device (forexample, a personal computer or portable terminal) additionally providedwith a photographing function. The camera includes the solid-state imagesensor according to the present invention, which has been exemplified inthe above embodiments, and a processing unit for processing a signaloutput from the solid-state image sensor. The processing unit caninclude, for example, an A/D converter, and a processor for processingdigital data output from the A/D converter.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2012-269778, filed Dec. 10, 2012, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A solid-state image sensor which includes asemiconductor layer having a first surface and a second surface,comprising: charge accumulation region pairs which are arranged betweenthe first surface and the second surface, one of the charge accumulationregion pairs including charge accumulation regions to accumulate acharge; a transfer gate which is arranged on a side of the first surfaceso that the transfer gate transfers the charge; microlenses which arearranged on a side of the second surface so that one of the microlensesis assigned to the one of the charge accumulation region pairs; a firstisolation portion which is arranged between the charge accumulationregions of the one of the charge accumulation region pairs, the firstisolation portion including an isolation region extending from thesecond surface toward the first surface; and a second isolation portionwhich is arranged between the charge accumulation region pairs, thesecond isolation portion continuously extending from the second surfacetoward the first surface, wherein a depth, from the second surface, ofthe isolation region of the first isolation portion is smaller than adepth, from the second surface, of the second isolation portion.
 2. Thesensor according to claim 1, wherein an impurity concentration of thefirst isolation portion is lower than an impurity concentration of thesecond isolation portion.
 3. The sensor according to claim 1, wherein awidth of the first isolation portion is smaller than a width of thesecond isolation portion.
 4. The sensor according to claim 1, whereinthe charge accumulation regions of the one of the charge accumulationregion pairs include a first charge accumulation region and a secondcharge accumulation region, and the transfer gate includes a firsttransfer gate transferring the charge of the first charge accumulationregion and a second transfer gate transferring the charge of the secondcharge accumulation region.
 5. The sensor according to claim 1, whereineach of the first isolation portion and the second isolation portionincludes a semiconductor region of a first conductivity type, and eachof the charge accumulation regions of the one of the charge accumulationregion pairs includes a semiconductor region of a second conductivitytype.
 6. The sensor according to claim 5, wherein the transfer gatetransfers the charge to a semiconductor region of the secondconductivity type.
 7. The sensor according to claim 5, furthercomprising a first semiconductor region of the first conductivity typearranged between the second surface and the charge accumulation regionsof the one of the charge accumulation region pairs.
 8. The sensoraccording to claim 7, further comprising a second semiconductor regionof the second conductivity type arranged between the first semiconductorregion and the charge accumulation regions of the one of the chargeaccumulation region pairs.
 9. The sensor according to claim 8, furthercomprising a third semiconductor region of the first conductivity typearranged between the first surface and the charge accumulation regionsof the one of the charge accumulation region pairs.
 10. The sensoraccording to claim 7, further comprising a film which is arranged on thesemiconductor layer and which contacts the first semiconductor region.11. The sensor according to claim 1, further comprising a first filmarranged between the microlenses and the semiconductor layer, a secondfilm arranged between the microlenses and the first film, and a lightshielding film arranged between the first film and the second film. 12.The sensor according to claim 1, wherein the second isolation portionincludes a first isolation region arranged between the first surface andthe second surface, and a second isolation region arranged between thefirst isolation region and the first surface.
 13. The sensor accordingto claim 12, wherein a width of the first isolation region of the secondisolation portion is larger on a side of the first surface than on aside of the second surface, and/or a width of the second isolationregion of the second isolation portion is larger on a side of the secondsurface than on a side of the first surface.
 14. The sensor according toclaim 1, wherein the isolation region of the first isolation portion isa first isolation region of the first isolation portion, the firstisolation region of the first isolation portion is arranged between thefirst surface and the second surface, and the first isolation portionfurther includes a second isolation region arranged between the firstisolation region of the first isolation portion and the first surface.15. The sensor according to claim 14, wherein a depth, from the firstsurface, of the second isolation region of the first isolation portionis larger than a depth, from the second surface, of the isolationregion.
 16. The sensor according to claim 14, wherein the secondisolation portion includes a first isolation region arranged between thefirst surface and the second surface, and a second isolation regionarranged between the first isolation region and the first surface, and adistance between the first isolation region and the second isolationregion of the first isolation portion is larger than a distance betweenthe first isolation region and the second isolation region of the secondisolation portion.
 17. The sensor according to claim 14, furthercomprising a semiconductor region having a conductivity type same as aconductivity type of the charge accumulation regions, arranged betweenthe first isolation region and the second isolation region of the firstisolation portion.
 18. The sensor according to claim 14, wherein thesecond isolation portion includes a first isolation region arrangedbetween the first surface and the second surface, and a second isolationregion arranged between the first isolation region and the firstsurface, and a dimension of the first isolation region of the secondisolation portion, in a direction from the first surface to the secondsurface, is smaller than a dimension of the second isolation region ofthe second isolation portion in the direction, and/or a dimension of thefirst isolation region of the first isolation portion, in a directionfrom the first surface to the second surface, is smaller than adimension of the second isolation region of the first isolation portionin the direction.
 19. The sensor according to claim 1, wherein thesemiconductor layer is provided with a shallow trench isolation.
 20. Acamera comprising: a solid-state image sensor according to claim 1; anda processing unit which processes a signal output from the solid-stateimage sensor, wherein by using the signal, the camera performs a focusdetection by a phase-difference detection method.